Photodynamic therapy (PDT) , which is the interaction of light with hematoporphyrin derivative (HDP, Photofrin II) , is in clinical use in several areas of application. Currently, PDT is being used clinically for a variety of skin lesions, bronchial obstructions, carcinoma in situ of the bladder, and intraperitoneal tumors. Animal models are being used to assess the potential of using PDT for lung mesothelioma. Specially designed fiber optic probes and introducers were manufactured for all cases, calibrated in terms of irradiance delivered to the tissue. Dosimetry for PDT is complex due to the variable irradiation levels within the tissue, sequestering of HPD, and tissue oxygenation. A plastic replica of a human lung was used to assess the light distribution across the wall for a variety of fiber optic probes and concentrations of light-scattering fluid.